structural geology, propagation mechanics and - Stanford School of ...

interpenetration occur, rather the boundary demarcating volume loss migrates outward
and into the surrounding material.
Central to the anticrack-inclusion concept, and the mechanical analyses to follow, is
the prescription of the model CB as an isolated feature of nonlinear inelastic strain that
evolves quasi-statically in an infinite, homogeneous, isotropic, linear elastic continuum
subject to uniform remote loading. All aspects of this model prescription warrant
comment. Firstly, given the regional tectonic nature of the compression and a study area
situated in the midst of a 1,400-m-thick sandstone deposit, the approximation of uniform
remote stresses applied to an infinite material is reasonable. Given a ratio of trace length
to spacing generally less than 0.1, it would be difficult to argue that any CB in the Aztec
sandstone is truly isolated in a mechanical sense. Nonetheless, the specific field data on
which the conceptual model is based comes from planar bands that betray little reaction
to their nearest neighbors.
As with any granular material, the applicability of homogeneous continuity is scale
dependent. In the Aztec sandstone, with an average grain diameter of 0.25 mm, this
becomes reasonable at the cm-scale (Amadei and Stephansson, 1997), which represents a
lower limit of resolution for interpreting the mechanical modeling results. To focus on
grain-scale processes inside and immediately outside a CB, the distinct element method
approach would be appropriate (Antonellini and Pollard, 1995; Morgan, 1999; Morgan
and Boettcher, 1999). Also problematic at a larger scale is the application of
homogeneous isotropy to the complex æolian sedimentary architecture of the Aztec.
Major dune boundaries influence CBs, so this conceptual model is based on data from an
outcrop of bands located within a single dune package. Given that CBs commonly cut
across depositional bedding without apparent effect, we interpret the mechanical
influence of such layering as negligible. Finally, the assumption of quasi-static CB
propagation is based on the coherent nature of plastic quartz grain deformation within the
bands, which suggests stable propagation accommodated by slow, visco-elastic relaxation
(Chester et al., 2004; Karner et al., 2003). By the same token, the paucity of plastic
deformation outside the bands suggests predominantly elastic behavior through time.
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